This invention relates to techniques for monitoring plasma processes used in semiconductor circuit fabrication, and more particularly relates to techniques for detection of various stages in such plasma processes.
Plasma processing is a well-established and accepted technology employed in the fabrication of semiconductor circuits. In particular, plasma etching techniques have become standardized processes for patterning semiconductor material layers. A plasma etch process typically involves the reaction of ionized reactant gases in a plasma state with portions of a material layer to be removed from a semiconductor wafer. Typically a patterned masking material is provided over portions of the layer to protect such portions from the reactant plasma gases, whereby the layer can be etched in a specific pattern during exposure to the reactive plasma gases.
A plasma etch process conventionally includes a series of stages such as pre-etch, main etch, and post-etch stages. A pre-etch stage includes, e.g., cleaning of the plasma chamber, striking of a plasma, and stabilization of a plasma; the main etch stage includes the material layer etch process, which may consist of multiple etch processes of differing chemistry; and the post-etch stage includes, e.g., an additional etch known as an over-etch process, and post-etch chamber cleaning. During each stage, the reactant gases introduced and ionized in the plasma chamber, as well as the product gases resulting from plasma reaction with the semiconductor material, interact with each other as well as with electrical and physical processes in a complex and nonlinear manner.
It has been demonstrated that characteristics of this complex plasma interaction are indicated in radiation emissions produced during the plasma process; the gases present in the plasma produce radiative emissions that are characteristic of the atomic and molecular species present in the chamber. Spectral analysis of the radiative emissions produced during an etch process have correspondingly been employed in known techniques for detecting the status of an etch process. A large effort has gone specifically to development of techniques employing optical emission analysis for detecting the main etch stage conclusion, known as the etch endpoint. Plasma etch endpoint is generally considered to be that point in time when the last traces of a layer being etched are removed. Optimally, the main etch stage is stopped just as the etching layer is removed and before underlying layers are damaged. Endpoint detection is thus a critical monitoring capability for successful plasma etching.
Plasma etch endpoint detection has been demonstrated with a range of techniques, the majority of which are based on monitoring of plasma radiation emission intensity at one or more wavelengths characteristic of the gaseous etch reactants and/or etch products associated with a main etch stage. When the monitored intensity changes in a prescribed manner with respect to a prespecified threshold intensity, etch endpoint detection is signaled. Other suggested detection techniques include, e.g., plasma impedance sensing.
When the material layer area being etched is not greatly exposed, i.e., when the etch pattern has a small exposed open area and a large masked area that is protected from the plasma, it is found that measurable changes in the emission intensity characteristic of the etch stage endpoint can be so small that analysis of the emission may not be meaningful. The radiative and electrical noise generated by the system can be so large as to swamp the measurable radiative emission. But as the linewidth of semiconductor devices continues to decrease, credible and reliable plasma etch endpoint detection for low open area etch patterns is critical. Further, as can be expected, the challenge posed by small linewidth devices for credible plasma etch endpoint detection also extends to credible monitoring of the other stages of the plasma etch process. Conventional plasma process monitoring techniques have been found to provide only suboptimal detection and monitoring results at small device linewidths, however.
It is also found that whatever device linewidth is to be etched, the condition of the plasma etch equipment can change over time due to, e.g., build up of deposits in the plasma chamber, so-called seasoning of the chamber, changes to semiconductor materials, and other factors, all of which can cause changes in what is detected as radiative emissions during the etch process. As a result, a satisfactory emission analysis for one etch process may be insufficient for a later etch process carried out under the same process conditions. Specifically, a static analysis prescription that cannot automatically consider changes in the plasma process environment can produce invalid process indications over time. This impediment, typical of conventional plasma process monitoring systems, is further worsened by small linewidth etch scenarios, leading to substantially suboptimal plasma process monitoring and control capabilities.